FREQUENCY-INDEPENDENT IcR ANTENNA

ABSTRACT

The invention provides interrupted coaxial-line radiators upon a ground plane and arranged in a frequency-independent manner for broadband radiation from a flush-mounted structure. The radiators may be arranged in a log-periodic array or in a spiral array for frequency-independent operation.

United States Patent Voronoff 1 Feb. 8, 1972 [54] FREQUENCY-INDEPENDENTICR ['56] References Cited ANTENNA UNITED STATES PATENTS vmnm' San3,480,961 11/1969 Copeland eta! ..343/771 [73] Assignee: Textron Inc.,Belmont, Calif.

Primary Examiner-Eli Lieberman 1969 Attorney-Gregg& Hendricson [21 1Appl. No.2 807,603

= [57] ABSTRACT I 52] us. (:1 ..343/792.5, 343/846, 343/895 Theinvention provides interrupted coaxial-line radiators p 51 1111.01..'..H0lq 11/10, HOlq 1/36 a ground plane and arranged in afrequency-independem [58! manner for broadband radiation from aflush-mounted struc Field of Search ..343/767, 771, 792.5, 895, 908

tnre. The radiators may be arranged in a log-periodic array or in aspiral array for frequency-independent operation.

7Clairns,5DrawingFigures PMENIEBFEB 81872 SHEEY 1 [IF 2 INVENTOR GEORGEN VORONOFF ATTORN EYS PATENTE DFEB a ma SHEET 2 {IF 2 5/! 52 53 32 FIG.4

I NV E NTO R GEORGE N. l/ORONOFF BY I ATTORNEYS 1 FREQUENCY- INDEPENDENTICR ANTENNA BACKGROUND OF INVENTION Requirements for broadband radiationfrom antennas has led to the development of what are generally termedfrequency-independent" antennas. In general, characteristics of anantenna of a given shape are dependent upon antenna dimensions measuredin wavelengths; thus antennas with several characteristic dimensions arenormally limited to operation within a fairly narrow band offrequencies. In order to overcome this limitation, it is possible toformulate antenna shapes described by angles rather than lineardimensions, and, in practical applications, these antennas-areessentially independent of frequency for all frequencies above a certainlower limit. This general class of antennas is herein termed frequencyindependent," and it is noted that this type of antenna may be definedin terms of polar coordinates with input terminals at the origin thereofwherein the antenna is bounded by curves which are equiangular spirals.The form of the bounding surfaces of such an antenna is fixed by therequirement that a change in scale should be equivalent to a rotation inazimuth angle,and this applies both to planar configuration andthree-dimensional shapes.

An alternative frequency-independent antenna configuration comprisesplane shapes that are, essentially cross sections of the generalthree-dimensional shape identified above. Although theelectricalproperties of such antennas are not strictly frequency independent, theydo repeat periodically with the logarithm of the frequency; consequentlyare generally termed log-periodic antennas. For this type ofantenna thefrequencies for which the structure has the same electrical propertiesare arranged in geometrical progression. Many different log-periodicantenna shapes are known, and various different log-periodic dipolearrays and log-periodic slot arrays have been developed. Reference ismade, for example, to'U.S. Pat. No. 2,989,749 setting forth one type ofanten na in accordance with this general proposition.

Further with regard to frequency-independent antennas, it is noted thatlog-periodic structures formed as dipole arrays are basically free-spaceantennas so as to be incapable of operation efficiently against a groundplane. Log-periodic slot antennas, on the other hand, requirecavitybacking, as set forth, for example, in U. S. Pat. No. 3,218,644.While this general classification of antennas will be seen to be highlyadvantageous, it is yet limited insofar as low-profile or flushmountedantenna structures are concerned. The present invention doesemploy theprinciples of frequency-independent antennas which may incorporatelog-periodic configuration but without the limitations noted above. I

Further with regard to the propagation of electromagnetic waves, therehas been developed a class of radiators commonly denominated asinterrupted coaxial-line radiators or slotted TEM-line radiators. Inthis respect, it is noted that the term IcR is oftentimes employed as anidentification of this type of radiator. A slotted TEM-line antenna isbasically a series-fed array of electrically small radiating loopelements, and in practice this normally embodied as a coaxial linehaving the sheath thereof transversely slotted along the length of theline to form the radiating elements. The length of transmission linebetween slots or radiating loops, as they may be termed, provides therequisite phase delay between elements of the antenna. Characteristicsof this type of antenna have been developed by prior workers in thefield, and a brief description thereof is to be found, for example, inIEEE transactions on antennas and propagation, Mar. 1969, page 260, aswritten by Mr. John R. Copeland. Radiation from this type of antenna mayoccur from the slot and/or from the center conductor at the slot but forconvenience such is herein termed slot radiation.

The present invention combines interrupted coaxial-line radiators infrequency-independent antenna configuration to achieve resultsunavailable with either alone. Although various antenna configurationsemployed in the present invention SUMMARY OF INVENTION In brief, thepresent invention provides a plurality of interrupted coaxial-lineradiators upon a ground plane and energized from input coaxial lineswith the radiators being oriented in frequency-independent array uponthe ground plane. Inasmuch as the ground plane is an integral part ofthe IcR, it is not necessary to back the structure with cavities;consequently, there is achieved a low-profile frequency-independentantenna, particularly suited for utilization upon aircraft,

spacecraft and the like. In accordance with one embodiment; of thepresent invention, the radiators of this antenna are arranged in alog-periodic fashion in two sections energized with like voltages ofopposite polarity to produce a linear principle polarization action of.electromagnetic radiation in space. Another embodiment of this inventionprovides interrupted coaxial-line radiators disposed logarithmic spiralor an Archimedean spiral with arms being. energized at the center of thespiral with like voltages of eith the same or opposite phaserelationship to produce a conical I circularly polarized beam with anull along a central axis normal to the ground plane, or a broadcircularly polarized beam with a peak along such axis.

The present invention is particularly directed to the provision of afrequency-independent antenna structure of lowprofile requiring nocavity backing for unidirectional radiation. The invention is applicablefor mounting upon metallic, nonplanar surfaces, such as those ofaircraft, missiles,and the antenna structure is particularly useful inthe VHF region.

DESCRIPTION OF FIGURES The present invention is illustrated as toparticular preferred embodiments thereof in the accompanying drawings,wherein:

FIG. 1 is a partial side elevational view of an interrupted coaxial-lineradiator;

- FIG. 2 is a transverse sectional view of an interrupted coaxial-Iineradiator taken in the plane 2-2 of FIG. 1;

FIG. 3 is a plan view of a log-periodic antenna in accordance with thepresent invention;

FIG. 4 is a schematic illustration of one section of the antenna of FIG.3; and

FIG. 5 is a perspective view of an Archimedean spiral antenna inaccordance with the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Considering first the generalaspects of .an interrupted coaxial-line radiator, reference is made toFIGS. 1 and 2 of the drawings. There is illustrated in these figures aconventional coaxial 'transrnissionline 11 having a central conductorI2, and a coaxial electrically conducting sheath 13. Many coaxial linesemploy a solid dielectric 1 4 between the central and outer conductorsthereof, and this may, for example, comprise solid polyethylenedielectric. In accordance with the present invention, the transmissionline 11 has spaced slots :16 formed transversely through the outersheath of conductor 13, with the central conductor 12 being continuousthroughout the line. In addition to the transmission line, as brieflydescribed above, there is provided an electrically conducting groundplane 21 upon which the line is disposed, in electrical contacttherewith. The slots 16 and the center conductor I2 at these slots actas radiating elements, and the slots may extend partially or entirelyabout the outer conductor 13. In the illustrated embodiment, the slotsare shown to extend about an angle which is somewhat less than theentire circumference of theouter conductor. Radiation from the line is,in part, dependent upon the circumferential extent of the slots therein,and, in addition, by forming the line as illustrated, it is possible toreadily attach the line to the ground plane, as by solder or in an-equiangular .or

welds 22, because a part of the sheath extends continuously along theground plane.

Electrical energization is provided to the transmission line 11, andelectromagnetic radiation emanates from the slots 16 therealong and fromthe center conductor exposed at these slots. A plurality of factors areinvolved in the characteristics of such radiation, as for example, theradii of the conductors,

the slot width and circumferential extent, as well as the voltageexisting across the slots and the distance between slots. Withoutentering into a detailed discussion of this type of radiator, it isnoted that the radiation characteristics thereof are known in the art,and for particular physical configuration can be calculated byconventional theory. The slotted transmission line 11 is termed aslotted TEM-line or an interrupted coaxial line radiator (lcR).

Referring now to FIGS. 3 and 4 of the drawing, there is illustrated onepreferred embodiment of the present invention, wherein interruptedcoaxial line radiators are employed, in accordance with the presentinvention, as radiators in a logperiodic antenna structure. The antennais formed of two portions 31 and 32, with these being fed by coaxiallines 33 and 34, respectively. The coaxial lines 33 and 34 are mountedupon an electrically conducting ground plane 36, and are fed orenergized by signals of equal amplitude but with 180 phase relationship,Energization of the coaxial lines may be accomplished from anappropriate transmitter, or the like, 37, including means such as ahybrid circuit for achieving the above-noted opposite phase relationshipof signals applied to the separate lines. The lines 33 and 34 areappropriately terminated as by a matched load therebetween.

Each'of the coaxial feedlines 31 and 32 is provided with branch-slottedTEM lines. It is to be appreciated that inasmuch as each portion mayhave a substantial number of branches, the illustration of FIG. 3 isbroken, so that only initial and terminal branches are illustrated forcompactness of illustration.

Referring to FIG. 4, there will be seen to be schematically illustratedthe orientation of branches of a single portion 31 of the antenna. Noattempt is made in FIG. 4 to show details of structure, but instead, theline diagram thereof is intended only to clarify the relative locationand extent of the branches of the section. In accordance withestablished theory of logperiodic antennas, the portion 31 illustratedin FIG. 4 will be seen to have a number of branches 41, 42, 43, 44,etc., extending from one side of the feedline 34, and a number ofintermediate branches 51, 52, 53, etc., extending from the other side ofthe feedline 34. Energy is fed into the coaxial line 34 at the leftthereof in the drawing and propagates along the line as well as out thebranch lines noted above. As this energy passes through the branchlines, electromagnetic energy thereof is radiated into space through thegaps in the lines as described above.

The embodiment of the present invention illustrated in FIG. 3 has thetwo portions 31 and 32 thereof formed as mirror images so that the firstbranch lines of each extend in opposite directions from each other andthe second branch lines extend toward and across each other. It isparticularly noted that the branch lines of each section alternate intheir direction of extension from the central feedline or coaxial cablethereof. It is also to be noted that in accordance with the theory oflogpen'odic antennas the length of successive branch lines 41 to 44, forexample, increases successfully from the feed end of the section, insomewhat the manner as illustrated in FIG. 4. In certain respects thismay be likened to a log-periodic dipole array except that interruptedcoaxial radiators are employed in place of dipoles. Furthermore, thesection 31 and 32 of the present invention are mounted upon a conductingground plane 36 in marked distinction to log-periodic dipole antennaswhich operate as free-space antennas. In circumstances where the antennais not operated at too high a frequency the crossing branches may beslightly offset to both pass over the other coaxial line.

The theory of frequency independency" of log-periodic antennas isapplicable to the present invention and consequently the relationship ofthe branch lines of radiators of the present invention is not furtherdeveloped herein. It is however, noted that the nature of the radiationpatterns and the radiation intensity from individual branch lines dependupon the termination of the end of the lines and the parameters of thegaps in the radiators. The branches may be terminated as short circuits,as illustrated, or alternatively may be open circuited or loaded. It isalso to be appreciated that the center conductor of the interruptedcoaxial line radiator need not be straight at the gaps therein. Theconductor may in fact be formed as a single or multiple loop if desiredin order to achieve additional control of radiation properties. Withregard to radiation pattern it is noted that the principal polarizationof the antenna of FIG. 3 is linear and the direction thereof in space isparallel to the branches or radiator lines. The orientation of principlepolarization of the antenna may be changed by changing the orientationof lines and in this respect, it is noted that these lines or branchesneed not be straight but may take other forms such as, for example,T-shaped or L-shaped It will be appreciated that once the desiredradiation characteristics of individual branches or radiator lines havebeen established the overall antenna pattern may be synthesized bywell-known methods.

An alternative embodiment of the present invention is illustrated inFIG. 5 wherein there is shown what may be termed an IcR Archimedeanspiral antenna. The antenna comprises a pair of parallel spiral coaxiallines or radiators 61 and 62 disposed upon a ground plane 63 in the formof an Archimedean spiral. The lines 61 and 62 are fed at the center ofthe spiral as by means of input coaxial cables 64 and 66 from a suitabletransmitter unit, not shown. The two arms 61 and 62 of this spiral areformed as interrupted coaxial line radiators, as indicated, with theinput portions 67 and 68 comprising impedance-matching sections. Thearms are energized from the center of the spiral, with energy then beingpropagated outwardly along the two arms 61 and 62 to the outer ends ofthe arms where they are appropriately terminated at a desired impedanceto to prevent reflection. The maximum spiral diameter is limited, inaccordance with established theory to the free-space wavelength atlowest operating frequency divided by twice the relative dielectricconstant in the loaded coaxial lines. It will be appreciated thatArchimedean spirals are essentially frequency independent and it isnoted that while these types of antennas are known in the art they havepreviously required cavity backing for unidirectional radiation.

The two arms of the antenna of FIG. 5 may be fed with input signals ofthe same or opposite phases depending upon the desired radiation patternfrom the antenna. Desired signal amplitude and phase relationships maybe achieved by utilization of a hybrid circuit at the input, forexample. In the instance wherein the two spiral arms 61 and 62 are fedin l phase relationship the antenna radiates a broad circularlypolarized beam with its peak directed along the axis normal to theground plane. Alternatively, when the two spiral arms 61 and 62 areenergized with signals of the same phase relationship the antennaradiates a conical circularly polarized beam with a null thereofdirected along the axis normal to the ground plane.

In common with the embodiment of the invention illustrated in FIG. 4 anddescribed above, the radiation patterns of the antenna of FIG. 5 areessentially independent of frequency provided that the gaps in theinterrupted coaxial radiators radiate a proper amount of power, asdiscussed above. This embodiment of the present invention does provideunidirectional radiation through the use of interrupted coaxial lineradiators in a frequency-independent array. In common with theabove-described embodiment of the present invention the presentembodiment has the advantages of a low profile and ability to conform tononplanar surfaces and is suitable for application in the VHF region.

Iclaim:

1. An improved antenna comprising an electrically conducting groundplane, a plurality of interrupted coaxial-line radiators disposed onsaid ground plane with the exterior of each radiator electricallycontacting said ground plane, said radiators being disposed infrequency-independent array, and a pair of coaxial cables disposed inadjacent parallel relation on said ground plane and connected toseparate radiators extending as stub branches from said cables inlog-periodic array on said ground plane for supplying electricalenergization to said radiators whereby electromagnetic energy isunidirectionally radiated into space from said radiators.

2. The antenna of claim 1 further defined by said cables carryingelectrical energization of opposite phase for supplyto said radiators.

3. The antenna of claim 1 further defined by each of said cables havingsaid stub branches extending from both sides thereof and alternatebranches of each extending across the adjacent cable as a coaxial cablewith the interrupted coaxial line commencing on the opposite side of thecable crossed.

4. The antenna of claim 1 further defined by said antenna having twomirror image portions with each including one of said cables and eachhaving branches extending alternately from opposite cable sides, saidbranches comprising said interrupted coaxial line radiators, and saidportions being disposed with the cables thereof in adjacent parallelrelation so alternate branches from the combination of portions extendoutwardly on each side thereof from different cables.

5. An improved antenna comprising an electrically conducting groundplane, a plurality of interrupted coaxial-line radiators disposed uponsaid ground plane in equiangular spirals with the exterior of eachradiator electrically contacting said ground plane, said radiators beingdisposed in frequency-independent array, a pair of coaxial cablesconnected to separate radiators at the center of said spirals forsupplying electrical energization thereto whereby electromagnetic energyis unidirectionally radiated into space from said radiators.

6. The antenna of claim 5 further defined by there being two spiral armsof radiators and said arms being energized with signals of like phase bysaid cables.

7. The antenna of claim 5 further defined by there being two spiral armsof radiators and said arms being energized with signals of 1 phaserelationship by said cables.

1. An improved antenna comprising an electrically conducting groundplane, a plurality of interrupted coaxial-line radiators disposed onsaid ground plane with the exterior of each radiator electricallycontacting said ground plane, said radiators being disposed infrequency-independent array, and a pair of coaxial cables disposed inadjacent parallel relation on said ground plane and connected toseparate radiators extending as stub branches from said cables inlog-periodic array on said ground plane for supplying electricalenergization to said radiators whereby electromagnetic energy isunidirectionally radiated into space from said radiators.
 2. The antennaof claim 1 further defined by said cables carrying electricalenergization of opposite phase for supply to said radiators.
 3. Theantenna of claim 1 further defined by each of said cables having saidstub branches extending from both sides thereof and alternate branchesof each extending across the adjacent cable as a coaxial cable with theinterrupted coaxial line commencing on the opposite side of the cablecrossed.
 4. The antenna of claim 1 further defined by said antennahaving two mirror image portions with each including one of said cablesand each having branches extending alternately from opposite cablesides, said branches comprising said interrupted coaxial line radiators,and said portions being disposed with the cables thereof in adjacentparallel relation so alternate branches from the combination of portionsextend outwardly on each side thereof from different cables.
 5. Animproved antenna comprising an electrically conducting ground plane, aplurality of interrupted coaxial-line radiators disposed upon saidground plane in equiangular spirals with the exterior of each radiatorelectrically contacting said ground plane, said radiators being disposedin frequency-independent array, a pair of coaxial cables connected toseparate radiators at the center of said spirals for supplyingelectrical energization thereto whereby electromagnetic energy isunidirectionally radiated into space from said radiators.
 6. The antennaof claim 5 further defined by there being two spiral arms of radiatorsand said arms being energized with signals of like phase by said cables.7. The antenna of claim 5 further defined by there being two spiral armsof radiators and said arms being energized with signals of 180* phaserelationship by said cables.